To get out of StarPilot-89 in order to use the math functions of the TI-89 calculator, you can do [2nd] [ESC] and then [2nd] [CUSTOM]. The first step gets you out of StarPilot, the second changes the menu bar to the TI version.

To get back into StarPilot, do [2nd] [ESC] then [green Diamond] [1]. Then you are back in StarPilot from just about anywhere you might be. If there is any chance that you have changed stored constants during your math work, then do Set Defaults Settings [4] [2] to reload StarPilot data.